Washington — With Wednesday’s Senate ratification of a new nuclear-arms reduction treaty with Russia, President Obama accomplished what he has said for months was his top foreign-policy priority.

The irony of Mr. Obama’s triumph is that, rather than constituting the dawn of a new era of measures reducing the nuclear threat, it may turn out to be the high-water mark in his efforts to rid the world of nuclear weapons.

In Wednesday’s vote, 13 Republicans voted for ratification. The Senate that takes office in January will be more Republican (though it will still have a Democratic majority), and a number of new senators will be markedly more conservative on foreign-policy issues like arms control, some political experts say.

“The 2010 elections changed the political landscape,” says Clifford May, president of the Foundation for Defense of Democracies and an opponent of passage of “New START” by a lame-duck Senate. A “reliable and modern weapons arsenal” and missile-defense systems –not weapons reduction – are likely to be the higher priority of a new, tea party-influenced Senate, he recently argued.

A harbinger of where arms-control debate could be headed may have been provided Wednesday by Mark Kirk (R) of Illinois, a new senator who has already taken his seat. He argued in a pre-vote statement on the Senate floor that the Obama administration should be “paying attention to ending the Iranian nuclear threat rather than to this agreement and its modest goals.”

Still, some arms-control experts predict that a new middle-ground consensus, forged by the New START debate, will find wind in its sails.

“Of course it’s going to be tougher with the new Republicans coming in, but it’s also not obvious that it means everything comes to a standstill,” says Daryl Kimball, executive director of the Arms Control Association in Washington. “What I see emerging from this debate is a bipartisan consensus that believes ... further effort needs to be pursued in reducing US and Russian warheads.”

New START modestly lowers the ceiling on the total number of nuclear warheads that the United States and Russia can possess – to 1,550 each. More broadly, both administration officials and many arms-control experts have said that approval of New START is essential if the US hopes to convince the world – including aspiring nuclear powers – that Obama’s vision of a nukes-free world is more than just rhetoric.

“The treaty’s ratification will reaffirm US leadership in reducing the global threat of nuclear weapons,” says Steven Pifer, director of the Arms Control Initiative at the Brookings Institution.

Some nuclear-nonproliferation advocates maintain that the issue of Iran, and how successful the US and other world powers are at stopping it from developing a nuclear weapon, will determine future steps at arms control. Indeed, one of the administration’s arguments in favor of New START was that it would further the good relations with Russia that have been crucial to Moscow’s cooperation in pressuring Tehran.

Mr. Kimball says New START’s ratification provides the “momentum” for the Obama administration to move forward on other elements of the nuclear-weapons-reduction agenda. He includes in that the long-stalled Comprehensive Test Ban Treaty, which Obama came into office pledging to ratify but which many conservative national-security experts oppose.

“Despite the opposition from what can only be called the Dr. Strangelove caucus,” Kimball says, “I think 70-plus votes [for New START] suggests the consensus is there to move forward, and I believe the administration will do just that.”